package/putty: fix build on uclibc

Fix build failure on uclibc raised since bump to version 0.75 in commit
d562009f

Fixes:
 - http://autobuild.buildroot.org/results/726f7c5ce13e78ed91e827b872e9d7ccfa13f298



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>